Optical fibers fall into two major categories, namely: step index optical fiber, which include single mode optical fiber and multimode optical fiber, and graded index optical fiber.
Single mode step index optical fiber has a core diameter less than 10 micrometers and only allows one light path.
Multimode step index optical fiber has a core diameter greater than or equal to 50 micrometers and allows several light paths, this leads to modal dispersion. Graded index optical fibers have their core refractive index gradually decrease farther from the centre of the core, this increased refraction at the core centre slows the speed of some light rays, thereby allowing all the light rays to reach the receiver at almost the same time, thereby reducing dispersion. Figure 2 gives a description of the various optical fiber modes.